1. Field of the Invention
The present invention relates to substrate including an oxide thin film transistor, and more particularly, to a substrate including an oxide TFT having improved initial threshold voltage degradation characteristics included in a driving circuit of a liquid crystal display (LCD) device, a method for fabricating the same, and a driving circuit for an LCD device using the same.
2. Description of the Related Art
A TV product has been the greatest application targets in a remarkably growing flat panel display device field. While an LCD has been the mainstream as a TV panel, an organic light emitting diode (OLED) display device, or the like, has also been intensively studied to be applied to a TV.
To meet the requirements, a substrate such as glass, or the like, is required to be increased, a display device having excellent performance without increasing cost, and a thin film transistor (TFT) to be applied as a switching and driving element are required.
An amorphous silicon TFT (a-Si TFT), a representative driving and switching element of a display device, is currently widely used as an element that may be uniformly formed on a large substrate having a size of 2 m at low cost.
However, as display devices tend to be increased in size and have high picture quality, an element is required to have high performance, so an existing a-Si TFT having mobility of 0.5 cm2/Vs level is determined to reach a limitation.
Thus, a technique of fabricating a high performance TFT having mobility higher than that of a-Si TFT is required. Also, the weakest points of a-Si TFT is that, as the a-Si TFT continues to operate, element characteristics thereof continues to be degraded, failing to maintain initial performance thereof.
Thus, currently, research into overcoming the limitation of the a-Si TFT has been conducted, and an oxide semiconductor TFT has been proposed as a typical TFT.
The oxide semiconductor TFT has high mobility relative to the a-Si TFT, so it is more advantageous in implementing a driving circuit for controlling a switching element of a liquid crystal panel provided in an LCD device, as well as the switching element.
FIG. 1 is an equivalent circuit diagram of a gate driving circuit of the related art LCD device, and FIG. 2 is a cross-sectional view illustrating a TFT of the gate driving circuit of FIG. 1.
The gate driving circuit of the related art LCD device includes a plurality of shift registers, and as shown in FIG. 1, the respective shift registers include a plurality of TFTs T1 to T8. In the respective shift registers, the first transistor T1 is diode-connected to be turned on by a start signal Vst to charge a Q node, and a sixth transistor T6 discharges a QB node by the start signal Vst. As the Q node is charged, a fifth transistor T5 is turned on to discharge the QB node. A second transistor T2 charges the QB node with a high potential driving voltage Vdd according to an inverting clock signal CLKB. As the QB node is turned on, a third transistor T3 is turned to discharge the Q node. A fourth transistor T4 is turned on by a reset signal RST to discharge the Q node and charge the QB node. A seventh transistor T7 is electrically connected to one side of the charged Q node and turned on by a high voltage charged in the Q node to allow a non-inverting clock signal CLK to pass therethrough so as to be output as an output signal Out. An eighth transistor T8 is turned on by the charged QB node to induce the clock signal CLK output through the seventh transistor T7 to be dropped to have a low potential.
As the thin film transistor of the shift register having the foregoing structure is implemented as an oxide transistor, performance thereof is anticipated to be enhanced according to high mobility. However, due to the characteristics of the oxide transistor, initial threshold voltages are irregular according to positions, and initial threshold voltages of some TFTs may be negatively shifted. In particular, an off-current level of the third transistor T3 connected to the QB node to which a high level voltage is continuously applied during a most driving time is increased.
Thus, a voltage level of the Q node connected to the third transistor T3 is lowered and a gate voltage of the sixth transistor T6 is lowered, and as a result, an output of the shift register is degraded.
In order to solve the problem, a method of improving defective driving due to negative shift by configuring the third transistor T3 to have a dual gate structure.
FIG. 2 is a view illustrating an example of a TFT having a dual gate structure applied to the related art shift register.
As illustrated, the TFT having a dual gate structure includes a first gate electrode 13 formed on an insulating substrate 10 having a dual gate structure, a first gate insulating layer 15 formed on the entire surface of the insulating substrate 10 including the first gate electrode 13, an active pattern 17 formed on an upper portion of the first gate insulating layer 15 overlapping the first gate electrode 13, an etch stop pattern 18 formed on the active pattern 17, source and drain electrodes 20 formed on the active pattern 17 in both sides of the first gate electrode 13, a protective layer 22 formed on the entire surface of the substrate including the source and drain electrodes 20, and a second gate electrode (or a back gate) formed to correspond to the first gate electrode 13.
In the TFT having a dual gate structure, a channel may be formed by using the second gate electrode 23, so a current flow through a back channel, as well as a current flow through a front channel of a TFT having a general back channel etched (BCE) invert-staggered structure, can be controlled to improve an off-current problem due to differences between the initial threshold voltages (initial Vth).
However, in order to apply the dual gate structure, an additional masking process is required to be added to form the second gate electrode 23, and also, since an additional control signal is required to be applied to the second gate electrode 23, a size of a driving circuit is increased. Thus, in case of the LCD device including the TFT having the dual gate structure, a fabrication process is complicated, an area of a driving circuit is increased, and fabrication cost is increased.